Fix incompatibility with Dokku v0.32.3

[DiegoMolero]

When running dokku with pre-build-buildpack I'm getting this error. Using pre-build fixes the issue

-----> Cleaning up...
-----> Building app from herokuish
remote:  !     Deprecated: please upgrade plugin to use 'pre-build' plugin trigger instead of pre-build-buildpack
-----> Adding shared deployment-keys to build environment ...
-----> Adding host-keys to build environment ...
       Adding shared keys
-----> Adding BUILD_ENV to build environment...
       BUILD_ENV added successfully
-----> Adding shared deployment-keys to build environment ...
remote: Unable to find image 'dokku/herokuish:latest' locally
remote: docker: Error response from daemon: pull access denied for dokku/herokuish, repository does not exist or may require 'docker login': denied: requested access to the resource is denied.
remote: See 'docker run --help'.
remote:  !     Removing invalid image tag dokku/app:latest
remote:  !     App build failed